The present invention relates to protective systems for use in AC electrical power distribution systems and more particularly to distance relays for AC electrical power transmission line protection.
Distance relays are well known in the art of protective relaying. For example, reference may be had to "The Art and Science of Protective Relaying", Mason, published by John Wiley and Son, Inc. (1956), particularly 115 to Chapter 14. Exemplary types of distance relays are disclosed in U.S. Pat. No. 4,034,269, issued to S. B. Wilkinson, on July 5, 1977; U.S. Pat. No. 4,420,788, issued to S. B. Wilkinson and G. E. Alexander on Dec. 13, 1983; and U.S. Pat. No. 4,405,966, issued to Leonardo Cavero, the inventor of the present invention, on Sept. 20, 1983. All these patents are assigned to the assignee of the present invention and are incorporated by reference in the present application as if fully set forth herein.
Distance relays are utilized in protective systems for AC power transmission lines to detect faults within protected zones or portions of the transmission lines. If such a fault is detected within the relay's zone or reach, the distance relay will enable operation of a circuit breaker in order to trip the faulted phase or phases.
Phase distance relays are utilized to detect phase to phase faults within a protected zone or portion of a transmission line. If such a fault is detected within the zone or reach of this type of distance relay, the phase distance relay will initiate a signal which will be used to cause the operation of a three pole circuit breaker to trip all three phases of the transmission line. This is to be contrasted with a ground distance relay which detects faults between a phase and ground, again within a designated zone or reach; and, upon detection of such a fault, generates a signal which enables the operation of a circuit breaker to trip the faulted phase only.
As indicated above, each particular distance relay should only detect faults within its protected zone or reach. The parameters of a distance relay are commonly selected to correspond to the parameters of the transmission line. For example, the parameters are selected to provide a forward reach that may correspond to the forward distance of the protected zone of transmission line under protection of the particular relay. It is desired that the distance relay operate within its selected forward reach thereby confining the protection of a particular distance relay to the selected zone within a protected system.
The desired operation of the distance relay may be hindered by the inability to distinguish between internal and external faults. Internal faults are hose which occur within the protected zone or reach of the relay, whereas external faults are those which occur outside the zone or relay reach. For example, errors in measuring fault resistance can cause the fault to appear further away from or closer to the distance relay location than it actually is; situations which, in the worse case, could prevent the operation of the distance relay on a fault occurring within its zone or reach, or could cause the relay to misoperate for a fault external to the protected zone or reach.
Although distance relays have, in the past, been used to detect faults within a predetermined protected zone or reach of the relay, such relays have not been able to provide a determination of the actual location of the fault; that is, the distance from the relay at which the fault occurred. The ability to determine the location of the fault in terms of distance from the relay enhances the utility of a distance relay.
Accordingly, it is an object of the present invention to provide a distance relay with improved discrimination between internal and external faults thereby increasing the reliability of the relay operation and the security of the protection system.
It is another object of the present invention to provide a distance relay for detecting faults in a protected zone of an AC electrical power transmission line, which relay determines the distance to the fault.
It is yet another object of the present invention to provide a distance relay with a capability for improved fault resistance determination.
It is a further object of the present invention to provide a distance relay which substantially eliminates the adverse effects on fault distance determination due to fault current components from multiple sources connected to the AC power transmission line.
It is still another object of the present invention to provide a distance relay which eliminates adverse effects of prefault load current on fault distance determination.
These and other objects of the present invention will become apparent to those skilled in the art upon consideration of the following description of the invention.